The present invention relates to digital wireless mobile communication systems employing a synchronization sequence, and more specifically, to methods and apparatus for estimating the delay spread in a time-varying multi-path fading channel.
Modem wireless mobile receivers are commonly implemented using digital signal processors. The signal of interest is first processed at the analog front-end circuitry, and is then sampled by an analog to digital converter (ADC). Subsequent processing is implemented on the stored digital samples, which are outputted from the ADC.
The propagation of a signal through a typical wireless channel generally results in a number of impairments on the received signal, such as multiplicative, dispersive, and additive imperfections. The dispersive imperfections introduced by multipath propagation may result in inter-symbol interference, which can require the use of equalization in the demodulation process. Demodulation refers to the extraction of transmitted message data from the samples of the received signal passing through the propagation channel.
Even in the presence of severe impairments, however, algorithms can be used by the digital signal processor to enable the reliable decoding of the transmitted data.
One important type of transmission impairment is known as delay spread, which is caused by the delay time of a signal being propagated over multiple paths (multi-path fading). This delay spread impairment is time-varying in a wireless mobile environment and causes interference between differently delayed versions of the same symbol, which arrive from different paths with varying delay intervals.
For many practical systems, the decoding to be performed in the case of minimal delay spread conditions is relatively simple. On the other hand, the decoding process in the presence of larger delay spread conditions requires the use of an equalizer scheme. However, the implementation of an equalizer scheme results in performance variations, and also further complicates the processing required, which translates to increases in instruction cycles and power consumption. Therefore, it is important to select the most efficient decoding scheme available for the particular multi-path channel environment.
In order to select the most appropriate decoding (demodulation) scheme, a delay spread estimation method is required. The delay spread estimation establishes the essential criterion upon which the selection of a demodulation technique is based.
Known delay spread estimation techniques are generally related to the estimation of either the variation of timing and conclusions based on the amount of this variation, or they involve performing the demodulation under different schemes, and concluding that the scheme with the best performance is most appropriate for the receive processing function.
One such scheme is disclosed in U.S. Pat. No. 5,400,368. However, the disclosed scheme does not take any advantage of the shape and roll-off of the correlation of the received signal with the synchronization pattern. In addition, the disclosed scheme does not take into consideration the effects of different degrees of delay spread conditions.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art through an improved method of delay spread estimation, which enables the optimal selection of a demodulation technique.
In accordance with an illustrative embodiment of the present invention, a method for estimating the delay spread of a received signal waveform comprises the following steps:
1) receiving a sampled segment of the signal waveform which corresponds to a predetermined synchronization sequence;
2) resampling the received sampled segment at a high sampling rate;
3) correlating the synchronization sequence with the resampled segment corresponding to different sampling times;
4) computing a correlation energy profile from these correlations, and determining the peak energy value and its position within the correlation energy profile;
5) computing correlation energy values at preselected equal offsets, before and after the position of the peak energy value;
6) determining the maximum of these offset correlation energy values, and computing a delay criterion, based on the ratio of this maximum offset correlation energy value to the peak energy value;
7) smoothing the data by averaging;
8) referencing the smoothed delay criterion to a predetermined lookup table; and
9) obtaining the estimated delay spread from the lookup table.
In most cases, it is desirable to know if the delay spread falls within a particular range. For this purpose, thresholds can be set up in the lookup table to identify delay spread ranges.
An illustrative embodiment of the present invention is more fully described below in conjunction with the following drawings.